The objective of this study is to determine the structure of enamel and the ultrastructure of the cells involved in amelogenesis in several mammals using both scanning and transmission electron microscopes. The purpose of this investigation is to compare enamel formation and enamel structure of mature teeth of man, monkey, cat, and dog in order to provide a basis for evaluating animal model systems and enhance our understanding of enamel structure (the rat is a widely used experimental animal model whose enamel formation and structure has been examined). Differences in enamel structure among these mammals will be noted, particularly if the enamel structure differs from that observed in human teeth. Emphasis will be given to uncovering basic principals of amelogenesis applicable to all enamel systems. Particular attention will be given to determine ameloblast function during enamel deposition in formation of Hunter-Schreger Bands and Lines of Retzius, and the role of papillary cells and their relationship to reduced ameloblasts. In the investigation of amelogenesis we will use both standard techniques for transmission microscopy and our approach of separating the apical region of the ameloblasts from the nascent enamel so that the enamel-ameloblast interface can be examined in the scanning electron microscope.